Pre-heat oven to 180 C
Line loaf pan with baking paper
Combine caster sugar & cinnamon for 'cinnamon sugar' in a bowl & set aside for later
Mix your dry ingredients in a bowl: flour, cinnamon, salt and baking powder
In a stand mixer with the whisk attachment, mix together the light brown sugar and buttermilk to combine
Slowly drizzle in your oil, with the mixer on, turning the speed to 'high' for 30-60 seconds to allow the oil to combine into the sugar/buttermilk mixture.
Keep mixing your wet ingredients on low, adding in the eggs & vanilla
Turn mixer to high for 30 seconds, ensuring all ingredients fully combined
Combine dry ingredients into wet - stir just enough to combine (don't over-mix here)
Fold in your chopped apples
Transfer cake mix to your lined loaf dish
Slice 1/2 apple thinly (3-4 slices) for decoration
Add decorative apple slices & cinnamon sugar mix to the top of the load
Bake for 55 minutes on 180 C
Remove & transfer to wire rack, allow to cool before slicing